(1) Field of the Invention
The present invention lies in the field of powering rotary wing aircraft having a plurality of engines, and more particularly to the field of regulating such a power plant.
The present invention provides a method of regulating a power plant for a rotary wing aircraft and also the power plant and a rotary wing aircraft provided with such a power plant. The invention is particularly intended for regulating a power plant having three engines.
(2) Description of Related Art
A power plant for a rotary wing aircraft generally comprises one or two engines and a main power transmission gearbox. Each engine drives the main gearbox mechanically in order to rotate at least one main outlet shaft of the main gearbox. The main outlet shaft is constrained to rotate with at least one main rotor of the rotary wing aircraft in order to provide the aircraft with lift and possibly also with propulsion.
The main gearbox generally also has secondary outlet shafts, e.g. for driving rotation of a tail rotor or of one or two propulsion propellers via an auxiliary gearbox, and also rotation of an electricity generator and/or hydraulic systems. The respective frequencies of rotation of the secondary outlet shafts are generally different from the frequency of rotation of the main outlet shaft.
It should be observed that the term “engine” is used to mean a driver unit driving said main gearbox mechanically, and consequently contributing to providing the rotary wing aircraft with lift and/or propulsion. By way of example, such engines may be turboshaft engines.
It is now common practice to use two-engined power plants on rotary wing aircraft, each engine being controlled by a dedicated computer. Such engines are generally identical turboshaft engines operating in compliance with regulation rules.
For example, proportional regulation can be used to enable a system to be regulated in proportion to a difference between a current value of the system that is to be regulated and a setpoint value. Such regulation is generally effective. However, proportional regulation never reaches the setpoint value, and there always exists a difference between the current value and the setpoint. Nevertheless, it is possible to approach the setpoint by reducing the difference, but the system then often becomes unstable.
Such proportional regulation, applied to a two-engined power plant of an aircraft enables the two engines of the power plant to be balanced naturally, both in terms of frequency of rotation and of power delivered. Nevertheless, such proportional regulation does not make it possible for the frequency of rotation of the main rotor of the aircraft to be stabilized accurately and effectively.
It is then possible to add a calculation for anticipating the power that the power plant is to deliver in order to improve the effectiveness of such proportional regulation of the frequency of rotation of the main rotor of the aircraft. Such power anticipation calculation is described in particular in Document FR 3 000 466 in the particular context of the main rotor having a frequency of rotation that is variable.
In order to improve proportional regulation, it is possible to introduce an additional correction that enables errors in tracking the setpoint to be eliminated. This correction is proportional to integrating the difference between the current value and the setpoint over time, i.e. it is proportional to the sum of all of the differences as measured continuously. This is then referred to as proportional integral regulation.
There also exists proportional integral derivative regulation that includes an additional correction that is proportional to the derivative of the difference. This correction makes it possible to take account also of variations in the difference, both in direction and in amplitude.
Proportional integral regulation is frequently used on twin-engined aircraft, thus making it possible to control accurately the frequency of rotation of the main rotor and also the performance of the aircraft. Operation is then balanced between the two engines of the power plant, thus making it possible in particular to ensure that wear is symmetrical on these engines and also on the mechanical inlet connections to the main gearbox.
However, such proportional integral regulation requires complex connections between the computers of the two engines in order to ensure that each engine delivers equivalent power. In particular, such proportional integral regulation requires the use of a balancing loop between the two computers.
Furthermore, the computers must be of relatively high performance in order to make such regulation possible. For example, these computers may be of the full authority digital engine control (FADEC) type. These computers are also often two-channel computers, i.e. the connections between the computers and also between the computers and the engines are duplicated in order to make those connections safe, and consequently make safe the operation of the power plant.
Furthermore, the size of rotary wing aircraft is tending to increase, so the need for power from the power plant is also increasing. Consequently, the power plants of such aircraft are being provided with at least three engines in order to be capable of delivering sufficient power.
Three-engined rotary wing aircraft are nowadays mainly fitted with three engines that are identical, thus making it possible in particular to ensure that the power plant responds reactively in the event of the failure of one engine, and also simplifying the installation and the integration of the engine.
Engines are said to be “identical” when they have identical drive characteristics for a rotary member.
Conversely, engines are said to be “unequal” when they have distinct drive characteristics, i.e. engines that generate different maximum powers and/or unequal maximum torque and/or different maximum frequencies of rotation of an outlet shaft. Thus, two unequal engines may correspond respectively to an engine driving an outlet shaft at several tens of thousands of revolutions per minute and to an engine driving an outlet shaft at less than ten thousand revolutions per minute, for example.
For a power plant having three identical engines, the three identical engines are generally regulated identically, with each engine delivering equivalent power.
Nevertheless, the regulation applied to the three identical engines can be different, e.g. two engines may be considered as main engines while the third engine is considered as a secondary engine. The secondary engine then delivers power that is additional to that delivered by the two main engines, depending on the loads on and the needs of the power plant. The power delivered by the secondary engine is then generally different from the power delivered by each of the main engines.
It is also possible to use unequal engines in a three-engined power plant, e.g. for the purpose of satisfying safety requirements or indeed of mitigating the lack of power from engines that are available on the market. For such a three-engined power plant, the regulation of the three engines can turn out to be even more complex, in particular in terms of distributing power among the engines and regulating the frequency of rotation of the main rotor.
In both situations, i.e. whether the engines of the power plant are identical or unequal, the distribution of power between the main engines and each secondary engine of the power plant can be problematic and difficult to optimize.
In particular Documents FR 2 998 542, FR 2 998 543, and FR 3 008 957 are known, which describe a power plant for a rotary wing aircraft having two identical main engines and a secondary engine.
Document FR 2 998 542 describes a secondary engine delivering constant secondary power, the secondary engine being put into operation under certain particular flight conditions such as landing, takeoff, or hovering.
In contrast, Document FR 2 998 543 describes a secondary engine delivering secondary power that is proportional to the main power delivered by each main engine with a coefficient of proportionality that is less than or equal to 0.5.
According to Document FR 3 008 957, the main engines are regulated on a first setpoint for the frequency of rotation of the main rotor of the aircraft, while the secondary engine is regulated on a second setpoint for the power of the secondary engine. Furthermore, the main engines are also regulated on a third setpoint for anticipated power so that the main and secondary engines acting jointly can deliver the power needed at the main rotor for the flight of the aircraft.
The dimensioning of the power plant of an aircraft is thus complex, independently of the selected configuration.
In the technological background, Document U.S. Pat. No. 4,479,619 is known, which proposes a power transmission system for three-engined helicopters. That solution also proposes an alternative to declutching of one of the three engines. The Super-Frelon helicopter of the Applicant also possesses three identical turboshaft engines.
Document U.S. Pat. No. 3,963,372 proposes a solution for managing power and controlling engines in three-engined helicopters.
In order to mitigate the problem of engines that are designed so as to be overdimensioned, a power plant having engines with unequal maximum powers, for two-engined aircraft, have already been envisaged in the past. This applies to Document WO 2012/059671, which proposes two engines having unequal maximum powers.
In addition, document FR 2 902 408 is also known that describes a fuel metering system for balancing power delivered by two turboshaft engines of an aircraft. For this purpose, a difference between the power margins of the two turboshaft engines is determined, then the turboshaft engine having the widest power margin is regulated so as to balance the power of the two turboshaft engines by minimizing said difference.
Finally, document GB 2 515 141 describes a system for synchronizing the performance margin of a power plant for an aircraft that is provided, for example, with two turboshaft engines. Firstly, a difference between the performance margins of the two turboshaft engines is calculated, then the two turboshaft engines are regulated so that said difference reaches a predetermined value.